1. Field of the Invention
The disclosure generally relates oil field equipment. More particularly, the disclosure relates to the blowout preventers.
2. Description of the Related Art
In gas and oil wells, it is sometimes necessary to close around or shear a tubular member disposed therein and seal the wellbore to prevent an explosion or other mishap from subsurface pressures. Typically, the oil field equipment performing such a function is known as a blowout preventer. A blowout preventer has a body that typically is mounted above a well as equipment in a blowout preventer stack.
A typical blowout preventer has a body with a bore through which a drill pipe or other tubular member can extend. Different types of rams, typically pipe, blind, or shearing sealing are associated with blowout preventers. Generally, a pair of rams extend laterally (that is, at some non-zero angle to the bore) from opposite sides of the bore. The rams are able to move axially within guideways and laterally to the bore. A pair of actuators connected to the body at the outer ends of the rams cause the rams to move laterally, and close around or shear the drill pipe disposed therebetween. A ram commonly have one or more sealing surfaces that seal against an object, the body of the blowout preventer, and an opposing ram. For example, shear blades on at least one type of ram are typically at slightly different elevations, so that one shear blade passes slightly below the other shear blade to cause the shearing action of an pipe or object disposed between the rams. After the shearing, sealing surfaces on the rams can seal against each other, so that the pressure in the well is contained and prevented from escaping external to the well bore.
Ram-type BOPs are required to have a device that mechanically locks and holds the sealing members of the BOP in the closed position. This mechanical lock acts as a safety precaution in the case of loss of hydraulic operating pressure to the actuators. The combination of accumulation of component tolerances and various sealing devices on the rams dictates that the mechanical locking device has to be able to lock the sealing components in a large range of positions. Thus, an axially movable lock is used.
Small, but important improvements thereafter characterized the industry. U.S. Pat. No. 4,076,208 to Olson is an example of the art that exists for these types of systems that have been used commercially. The Abstract states, “A new and improved ram lock for blowout preventer rams which permits locking of the ram at multiple and adjustable positions to compensate for wear on sealing elements of blowout preventer rams and increase sealing action of the ram without requiring separate special control lines. Automatic locking of the ram at a desired position, such as in adjustable sealing positions to compensate for ram elastomer wear, is obtained.”
Olson acknowledges that prior art U.S. Pat. No. 3,242,826 and Re27,294 teaches snap rings or collets that locked when the piston in the actuator reached a predetermined locking position, but offered only one locking position. Olson also acknowledges that U.S. Pat. No. 3,208,357 teaches a blowout preventer with a taper locking pin, but requires extra hydraulic operating and control lines, increasing the complexity of the control system. In Olson, the text and figures teach a lock that is activated by engagement of ratchet teeth 38a, 42a on engaged clutch plates 38, 42, as illustrated in FIGS. 2, 4. Springs 40 maintain contact between the ratchet teeth 38a, 42a during rest and during inward movement of the ram toward the center of the BOP. As the ram moves inwardly and the lock nut 40 rotates, the ratchet teeth 38a, 42a incrementally index relative to each other, allowing ram movement inwardly, but locking ram movement outwardly by locking the nut 40 from reverse rotation. To release the ratchet teeth so that the ram can move in the reverse direction outwardly, an annular unlocking piston 62 in an unlocking chamber 60 is actuated and forces the clutch plates and ratchet teeth apart, as shown in FIG. 2A. The pressure on the unlocking piston is the opening or clutch fluid pressure in the cylinder 17 from fluid entering through the inlet conduit 24b. Such fluid is conveyed at such pressure to the unlocking chamber 60 to actuate the unlocking piston 62, as the ram begins to movement outwardly.
However, such a locking system may be subjected to high loads when the BOP is closed and opened, which may reduce the service life of the locking system. The system described by Olson applies the opening fluid pressure to the unlocking piston, when such opening fluid pressure is also applied to the ram carrier to try to force the ram outwardly. Thus, the pressure on the ram carrier can increase the force between the ratchet teeth to resist such force, because the ratchet teeth are locked together to prevent the ram from moving outwardly. While the ratchet teeth are locked together under such force, the unlocking piston is trying to overcome such force to disengage the ratchet teeth. Thus, the system causes such parts to oppose each other's intended movement until the unlocking piston can disengage the ratchet teeth to allow the outward movement of the ram. The system can create wear on the ratchet teeth as the ratchet teeth engage and disengage each other. Further, the ratchet teeth are continually engaged during the inward movement of the ram, further contributing to potential wear on the system.
Therefore, there remains a need for improved locking system for a blowout preventer that reduces the potential wear on the locking system and activates the clutch engagement in a more controlled manner.